Refactor tool output format

internal/llm/client_tools.go

@@ -2,11 +2,10 @@ package llm
 
 import (
 	"context"
-	"fmt"
-	"log/slog"
 	"runtime"
-	"strings"
 	"sync"
+
+	"log/slog"
 )
 
 // callTools executes each tool call in the message concurrently and returns
@@ -41,26 +40,31 @@ func (c *Client) callTools(ctx context.Context, msg Message) []Message {
 					slog.Any("error", err),
 					slog.String("name", tc.Function.Name),
 				)
-				result = fmt.Sprintf(
-					`{"ok": false, "error": %q}`, err,
-				)
-			} else {
-				c.log.InfoContext(
-					ctx,
-					"called tool",
-					slog.String("name", tc.Function.Name),
-				)
+				results[idx] = Message{
+					Role: RoleTool,
+					ContentParts: []ContentPart{{
+						Type: ContentTypeText,
+						Text: err.Error(),
+					}},
+					Name:       tc.Function.Name,
+					ToolCallID: tc.ID,
+				}
+				return
 			}
 
-			// Content cannot be empty.
-			if strings.TrimSpace(result) == "" {
-				result = `{"ok": true, "result": null}`
-			}
+			c.log.InfoContext(
+				ctx,
+				"called tool",
+				slog.String("name", tc.Function.Name),
+			)
+
+			formatted := result.FormatResult()
 
 			results[idx] = Message{
 				Role: RoleTool,
 				ContentParts: []ContentPart{{
-					Type: ContentTypeText, Text: result,
+					Type: ContentTypeText,
+					Text: formatted,
 				}},
 				Name:       tc.Function.Name,
 				ToolCallID: tc.ID,